The present invention relates to cooling a microchip on a circuit board.
Electronic apparatuses usually comprise one or more circuit boards, in particular printed circuit boards, each supporting one or more microchips. High performance microchips produce heat, which has to be transported away from the microchips to prevent damage of the microchips and to achieve a high lifetime for the microchips. To this aim a modern apparatus comprises a cooling device, in particular based on fluid or water cooling. Such a cooling device usually has a cooling plate, on which the circuit board is mounted.
Higher performance generally leads to rising heat output, therefore requiring better cooling.
U.S. Pat. No. 6,444,496 discloses usage of a perform for introducing thermal paste into semiconductor packages.
U.S. Pat. No. 5,757,621 discloses a heat sink assembly, wherein a viscous fluid as thermally conductive compound is applied between an electronic device and the heat sink.
It is an object of the present invention to provide an improved cooling. The object is solved by the independent claims. Preferred embodiments are shown by the dependent claims.
The mounting of the microchip at that side of the circuit board, that is facing the cooling device, provides a very short path for the heat transfer. Accordingly, the cooling of the microchip performs with a higher efficiency.
A microchip is provided having an electronically passive side supporting an electronically active side. Said active side comprises all integrated circuits, electronic elements like transistors, diodes etc., and therefore produces heat during operation. A direct cooling of the active side is not advisable, as the active side is very touchy. By arranging the microchip with its electronically active side facing the circuit board and with its electronically passive side turned to the cooling device, the cooling can be enhanced further. This special arrangement takes into consideration that on one hand the heat producing active side is strongly and thermally conducted with the passive side, and on the other hand that the passive side is not touchy and therefore allows direct cooling. The preferred embodiments consequently use this cognition to improve the heat transfer path and to increase the cooling.
According to manufacturing tolerances there may occur a gap in-between the microchip and the cooling device. Such gap represents a very high thermal resistance in the heat transfer path from the microchip to the cooling device. Therefore, in another embodiment the cooling can be enhanced by means of a thermal conductor, which is arranged between the microchip and the cooling device and which thermally contacts the respective side of the microchip, preferably its passive side, with the cooling device. This thermal conductor fills out the gap and consequently improves the heat transfer from the microchip to the cooling device.
Since the aforementioned manufacturing tolerances might lead to gaps with different distances between the microchip and the cooling device, the thermal conductor is preferably designed to cover the largest possible gap. If smaller gaps occur, the thermal conductor might have to be squeezed. This squeezing may result in reset forces leading to impermissibly high tensions in the circuit board. To prevent damage of the circuit board an advanced embodiment suggests the use of a thermally conductive and kneadable or plastically deformable material, e.g. thermally conductive knead or plasticine, as the thermal conductor. Such a material, like knead or plasticine, can be deformed easily, wherein this deformation is mostly performed plastically and hardly elastically, such that after the deformation the material shows practically no reset forces. Since the proposed thermal conductor is kneadable, it can easily be adapted to each possible gap, wherein the missing reset forces prevent high tensions in the circuit board and therefore prevent harming the circuit board.
According to the viscosity of the kneadable thermal conductor its deformation requires either relative high deformation forces to reach a short deformation time or a relative long deformation time by using small forces. Since circuit boards are mounted on the cooling device automatically in the scope of a mass-production, the mounting has to be performed in a time too short for individually adapting the thermal conductors to the respective gaps. A preferred embodiment overcomes this difficulty by mounting the circuit board on the cooling device in a way that the circuit board or at least a area of the circuit board, which supports the microchip, is loosely secured to the cooling device, i.e. the circuit board or at least its microchip supporting area is with respect to the cooling device movably attached to the cooling device. Furthermore the circuit board or at least its microchip supporting area is pre-stressed against the cooling device with a predetermined pre-stress force, which can be designed according to the requirements of the deformation of the thermal conductor and according to the elasticity of the circuit board. Therefore, impermissible high tensions in the circuit board can be prevented. This pre-stress force and the said loose attachment of the circuit board are achieved by means of a pre-stress device, which preferably comprises at least one spring member and at least one support member. The support member is fixedly mounted on the cooling plate and supports the spring member pre-stressing the printed circuit board against the cooling plate with the predetermined or designed pre-stress force.
A preferred embodiment of the present invention is used in an automated test equipment (ATE) for testing or verifying Integrated Circuits (IC). ICs generally need to be tested to assure proper operation. Thisxe2x80x94in particularxe2x80x94is required during IC development and manufacturing. In the latter case, the ICs are usually tested before final application. During test, the IC, as device under test (DUT), is exposed to various types of stimulus signals, and its responses are measured, processed and usually compared to an expected response of a good device. Automated test equipments (ATE) usually perform these tasks according to a device-specific test program. Examples for ATE are the Agilent 83000 and 93000 families of Semiconductor Test Systems of Agilent Technologies as disclosed e.g. under http://www.ate.agilent.com/ste/products/intelligent test/SOC test/SOC Tech Oview.shtml. Details of those families are also disclosed e.g. in EP-A-859318, EP-A-864977, EP-A-886214, EP-A-882991, EP-A-1092983, U.S. Pat. No. 5,499,248, U.S. Pat. No. 5,453,995. The teaching of those documents shall be incorporated herein by reference.